1. Field of the Invention
An aspect of the present invention relates to a nonvolatile semiconductor storage device and a method for manufacturing the same.
2. Description of the Related Art
NAND flash memory has a structure in which a plurality of nonvolatile semiconductor memory elements are connected in series. For example, a single element has a source region and a drain region formed on a surface of a semiconductor substrate, and a gate insulating film, a floating gate, an inter-gate insulating film, and a control gate that are sequentially stacked on a channel region sandwiched between the source region and the drain region. In a miniaturization of a nonvolatile semiconductor memory element using a floating gate, there are pointed out that it is difficult to reduce the height of the floating gate, that it is difficult to perform ion implantation for forming a diffusion layer or a channel, and that interference between transistors occur.
A MONOS (Metal-Oxide-Nitride-Oxide-Silicon) or SONOS (Silicon-Oxide-Nitride-Oxide-Silicon) type nonvolatile semiconductor storage device is available for solving the above-mentioned points resultant from pursuit of miniaturization of the structure (see JP-2005-268756-A). A term MONOS type will be used hereunder on behalf of the MONOS type and the SONOS type.
The MONOS-type nonvolatile semiconductor memory element is provided with an insulative charge storage layer formed of a silicon nitride film in place of a floating gate of a related-art structure. The nonvolatile semiconductor memory element is constituted of a source region and a drain region formed on the surface of a semiconductor substrate, and a tunnel insulating film, a charge storage layer formed of a silicon nitride film, an interlayer insulating film, and a control gate that are sequentially stacked on a channel region sandwiched between the source region and the drain region. In NAND-type flash memory, the above-mentioned points of the floating-gate-type nonvolatile semiconductor memory element can be solved by the MONOS-type nonvolatile semiconductor memory element.
However, in the MONOS-type element structure, it is difficult to improve a writing characteristic.